Overview of TNM: TNM computes highway traffic noise at nearby receivers and aids in the design of highway noise barriers. As sources of noise, it includes 1994-1995 noise emission levels for the following cruise-throttle vehicle types:

Automobiles: all vehicles with two axles and four tires -- primarily designed to carry nine or fewer people (passenger cars, vans) or cargo (vans, light trucks) -- generally with gross vehicle weight less than 4,500 kg (9,900 lb);

The development work of Harris Miller Miller & Hanson, Foliage Software Systems, Vanderbilt University and the University of Central Florida was conducted in part under contract to Foster-Miller, Inc. Vanderbilt University and the University of Central Florida were also under contract to the Volpe Center.

1.2 Free Field Levels (TNM V2.5 UPDATE SHEET)

Characteristics of the free-field noise level computations include:

TNM computes three different sound-level descriptors, depending on user selection: the energy-equivalent sound level over a one-hour time period (1HEQ, represented by the symbol, LAeq1h), the average day-night sound level (DNL, represented by the symbol, Ldn), or the average day-evening-night sound level, designated as the Community Noise Equivalent Level (CNEL, represented by the symbol, Lden)2

Traffic control devices can be inserted, and the TNM computes vehicle speeds and emission levels accordingly. Such devices include traffic signals, stop signs, toll booths, and on-ramp start points.

Computations are performed in a-octave bands for increased accuracy; this aspect is not visible to users.

The TNM computes noise contours if specified; the NMPLOT Version 3.05 contouring program is used for compatibility with the Federal Aviation Administration's Integrated Noise Model (INM) Version 5.0 and higher [Olmstead 1996], and the U.S. Air Force's NOISEMAP program [Moulton 1990].

More details on the computation of vehicle speeds are given in Section 2.2 and Appendix B of this manual; details on the computation of free field levels are given in Section 2.3 and Appendix C.

1.3 Shielding and Ground Effects (TNM V2.5 UPDATE SHEET)

The TNM incorporates state-of-the-art sound propagation and shielding algorithms. These algorithms are based on fairly recent research on sound propagation over ground of different types, atmospheric absorption, and the shielding effects of barriers, berms, ground, buildings, and trees. The TNM does not account for atmospheric effects such as varying wind speed or direction or temperature gradients. The TNM propagation algorithms assume neutral atmospheric conditions. Characteristics of the propagation algorithms include:

Berms can be defined, with the user-selectable heights, top widths and side slopes; they are computed as if they were terrain lines. Berms can also be defined with top-widths in versions of TNM 2.1 and earlier, but are limited to a top-width of 0 meters (or feet) for TNM 2.5, because of apparent diffraction algorithm anomalies associated with flat top berms in all versions of TNM. For documentation purposes, descriptions of the acoustics and functionality associated with flat top berms remains in this manual, in case they are implemented in future versions of TNM.

Rows-of-buildings attenuation is included, with user-definable height and percentage of area blocked relative to the source roadway(s).

Tree zones can be defined; the ISO standard for attenuation by dense foliage is used [ISO 1996].

Multiple reflections between parallel barriers that flank a roadway are computed in two dimensions, unlike other TNM acoustics, which are computed in three dimensions. This is discussed further in Section 1.5 and Appendix E. Reflections in TNM outside of the parrallel barrier module are disabled in all versions of TNM. Descriptions of the acoustics and functionality associated with reflections remains in this manual for documentation purposes, in case they are implemented in future versions of TNM.

Double-barrier diffraction is included. The net effect of diffraction from the most effective pair of barriers, berms or ground points that interrupt the source-receiver line of-sight is computed. The other objects that interrupt the path are ignored.

More details on the computation of shielding and ground effects are given in Sections 2.4, 2.5, 2.6, and Appendix D of this manual.

1.4 High-level Flow Chart (TNM V2.5 UPDATE SHEET)

This section presents a flow chart to outline the overall flow of the TNM during sound level calculation. It is presented as Figure 1.

1.5 Parallel Barrier Analysis(TNM V2.5 UPDATE SHEET)

A two-dimensional multiple-reflections module has been included within the TNM for computing the degradation of barrier performance due to the presence of a reflective barrier on the opposite side of the roadway. The results from this module are generalized by the user to modify the TNM's results where multiple reflections exist. The module is most effective in computing the effects of sound-absorbing material on the surfaces of barriers or retaining walls. More details on the parallel barrier module are given in Appendix E of this manual.

An additional field study was undertaken to determine the effective source heights of various vehicles [Coulson 1996]. This study assigned two "sub-source" heights to each vehicle type. They are 0 meters (0 feet) and 1.5 meters (5 feet) above the pavement for all vehicles except heavy trucks, where the upper source is 3.66 meters (12 feet) above the pavement. The study also determined the ratio of sound energy distributed at the lower and upper heights as a function of frequency, vehicle type, and throttle condition (cruising or full throttle). Table 1 shows the percentage of total emission sound energy distributed to the upper source height at the low frequencies and at the high frequencies. In the middle frequency range, between 500 and 2000 Hz, the sound energy distribution is presented in Appendix A, including curves showing the sound energy split by frequency for each vehicle type.

Further detail about the energy distribution is presented in Appendix A, Section A.4.4

2.2 Vehicle Speed Computation (TNM V2.5 UPDATE SHEET)

The TNM computes adjusted speeds based on the user input speeds, roadway grade, and traffic control devices. For level or down-grade roadways, TNM uses the speeds assigned to the roadway by the user (the "input speed"). For heavy trucks (only) on upgrades equal to 1.5 percent or more, TNM reduces the input speeds. The speeds are reduced depending on the steepness and length of the upgrade in accordance with speed-distance curves similar to those published for geometric design by the American Association of State Highway and Transportation Officials [AASHTO 1990 and TRB 1985]. The TNM speed-distance curves were calibrated to the speeds measured during the emission level noise measurement program. Appendix B describes the details of these computations and gives examples.

The TNM allows the user to enter the following traffic-control devices: traffic signals, stop signs, toll booths, and on-ramp start points. The reason for these devices is to allow a more precise modeling of vehicle speeds and emission levels under these interrupted-flow conditions. TNM will compute speeds all along any roadways with traffic control devices. These devices abruptly reduce speeds to the device's "speed constraint," for the device's "

3Note: The values in this table for autos, medium trucks, buses and motorcycles have been corrected; they were previously the ratio of upper to lower subsource heights, rather than the precentage of total sound energy at the upper subsource height. For heavy trucks, 20% more energy has been shifted to the upper subsource height.

Table 6. Constants for subsource-height split.

Vehicle type, i;

Pavement type, p

Full
throttle

Constants, For a user-defined vehicle, use the TNM equivalent vehicle to choose the relevant table row for these five constants

Au

MT

HT

Bus

MC

Avg

DG AC

OG AC

PCC

Yes

No

L

M

N

P

Q

X

X

X

X

X

X

X

0.373239

0.976378

-13.195596

39.491299

-2.583128

X

X

X

X

X

X

0.579261

0.871354

-177.249214

558.980283

-0.026532

X

X

X

X

X

X

0.566933

0.93352

-25.497631

80.239979

-0.234435

X

X

X

X

X

X

1.330000

0.08000

-204.84400

592.56800

-159.34400

X

X

X

X

X

X

0.850000

-0.33000

163.021000

-492.45100

-58.00500

X

X

X

X

X

X

0.579261

0.871354

-177.249214

558.980283

-0.026532

X

X

X

X

X

X

0.563097

0.928086

-31.517739

99.099777

-0.263459

X

X

X

X

X

X

0.391352

0.978407

-19.278172

60.404841

-0.614295

X

X

X

X

X

X

0.391352

0.978407

-19.278172

60.404841

-0.614295

A.4.2 User-defined vehicles (TNM V2.5 UPDATE SHEET)

For a user-defined vehicle, TNM substitutes the subsource heights for the built-in vehicle that the user designates as most similar. Table 6 mentions this substitution in the appropriate column heading.

A.5 Vertical Subsources, Free Field (TNM V2.5 UPDATE SHEET)

Next TNM eliminates the ground effects within these measured vehicle emissions. To do this, it multiplies each measured vertical subsource emission by the values in Table 7.

Mathematically:

Equation 8

The subscripts, ff, stand for free field. Physically, this last equation represents each vehicle type's measured energy-mean emission spectrum, as if the vehicles passed by during measurements at 15 meters (50 feet) without any intervening ground (that is, free field).

Table 7. Multiplier, m, for each built-in subsource height.

Freq (Hz)

50

63

80

100

125

160

200

250

315

400

500

630

Multiplier m, Height: 3.66m

0.30

0.32

0.36

0.44

0.52

0.69

0.95

1.78

1.00

0.32

0.40

0.25

Multiplier m, Height: 1.5m

0.26

0.27

0.27

0.28

0.30

0.33

0.38

0.48

0.62

0.79

1.12

1.58

Multiplier m, Height: zero

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.25

0.20

Freq (Hz)

800

1000

1250

1600

2000

2500

3150

4000

5000

6300

8000

10000

Multiplier m, Height: 3.66m

0.25

0.25

0.25

0.25

0.32

0.56

1.00

1.00

1.00

1.00

1.00

1.00

Multiplier m, Height: 1.55m

0.40

0.50

0.32

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

Multiplier m, Height: zero

0.25

0.25

0.22

0.20

0.25

0.27

0.34

0.42

0.47

0.52

0.59

0.67

These values were derived by using propagation algorithms of TNM to determine the effect of the (absorptive) ground present during the emission-level measurements.

D is multiplied by a sign function that is positive when the receiver is in the dark zone and negative when the receiver is in the bright zone. To adjust the diffraction field to make it consistent with empirical results, D is also multiplied by an adjustment factor, A. A is currently set to 1.2. The factor Q is included to account for the surface impedances at the diffracting edge (see Section D.4.5). This results in the following equation:

Equation 31

Chi function:The chi (χ) function is used to pass information about the diffracting geometry to the Fresnelfunction. It takes into account the distances from the diffraction point for the effective source and the receiver, the angle formed about the diffraction point, and the top angle of the obstruction causing the diffraction. The χ function has the following formula

APPENDIX G MODEL VERIFICATION (TNM V2.5 UPDATE SHEET)

This appendix provides a comparison of TNM 1.0 results to measurements and to the model results of others. Comparisons are made to five different data sets, three of which involved point-source geometry, and the remaining two involved in-situ measurements of barrier performance along actual highways. The first comparison is with Embleton's model for reflection from ground of finite impedance [Embleton 1983]. The second is to measurements by Parkin and Scholes over grassland [Parkin 1965], the third is to measurements of a noise barrier by Scholes, also over grassland [Scholes 1971]. The fourth and fifth are to measurements of noise barrier performance at two different highway locations by Hendriks and Fleming, respectively [Hendriks 1991] [Fleming 1992]. Overall, the agreement with measurements is found to be very satisfactory.

Comparisons of results from later versions of TNM to measurements are presented in the TNM validation report [Rochat 2002]. An addendum to this report reviewing the performance of TNM 2.5 will be published in 2004.

G.1 Ground Reflection Model

The TNM's model for reflection coefficients is based on the approach of Chessell [Chessell 1977], which incorporates the single-parameter ground-impedance model first proposed by Delany and Bazley [Delany 1970]. Embleton, Piercy and Daigle further developed the model and conducted measurements to determine empirically the relationship between ground type and effective flow resistivity (EFR) [Embleton 1983]. Figures 71 through 74 present a comparison of the TNM model with Embleton's model for Embleton's published geometry and four values of EFR. The geometry was: source height = 0.31 meters (1.0 feet); receiver height = 1.22 meters (4.0 feet); source-to-receiver distance = 15.2 meters (50 feet). The values of EFR span the range from very soft ground (powder snow, EFR = 10 cgs Rayls) to hard ground (10,000 cgs Rayls).

Plotted in the figures are values of the "ground effect" in dB, which represents the difference between the free-field (no-ground) condition and the condition with the ground. At low frequencies, the ground adds up to 6 dB, due to pressure doubling. In the middle frequencies and over soft ground (EFR = 100 to 500) the fairly broadband "ground-effect dip" exhibits significant reductions in sound level due to destructive interference.

G.2 Measurements Over Grassland

The TNM's reflection model is compared with very carefully conducted measurements of sound propagation over grassland by Parkin and Scholes [Parkin 1965]. The atmospheric conditions for the measurements were a normal temperature gradient (no strong lapse or inversion) and zero vector wind (no components in the source-to-receiver direction). The ground surface at the site, called "Hatfield," was grass up to 5 centimeters (2 inches) high covering silty soil. The ground was especially flat, within ± 0.3 meters (1 foot) for more than 500 meters (1500 feet). The source was a jet engine at a height of 1.8 meters (6.0 feet) and the microphone heights were all 1.5 meters (5.0 feet) above the ground. One-third octave band sound level measurements were made at the following distances: 35 m (114 ft), 62 m (202 ft), 110 m (360 ft), 195 m (640 ft) and 348 m (1140 ft).